In the prior art primarily crucibles of silicon dioxide (quartz ceramics) are used for these purposes. In the crucibles of the prior art, the material to be melted is completely melted. To prevent leakage of the melt, these crucibles are constructed monolithically, i.e. frame and bottom plate are firmly joined together. In other words, crucibles of the prior art are typically made of silicon dioxide, whereby the thermal conductivity toward the bottom and towards the sides is equal. The crucibles used in the prior art require a massive increase in the temperature gradient toward the main heat flow during the solidification process. This massive increase of the temperature gradient reduces the degrees of freedom in the configuration of the crystallization process. In addition, unwanted crystal growth can be induced, which deviates from the main direction of growth and can reduce the yield of high quality material particularly in the field of the production of crystalline silicon. In the prior art, usually the Bridgman method or VGF-(Vertical Gradient Freeze) technology is used for the production of crystalline silicon ingots.
Particular attention must be paid to ensure that the melt is not contaminated by impurities. In the case of silicon for photovoltaics this would lead to a deterioration in efficiency and to a reduced yield.
Another major disadvantage of the crucibles of the prior art is that the entire crucible is damaged during the process by phase transitions and the associated changes in volume. Currently, the crucibles used must be loaded manually very carefully and cautiously. In the case of incorrect production and/or improper loading of the crucible damaged spots can result in failure and thus consequently to leakage of the melt already during the melting process. The further increases in the batch sizes to be expected and the associated crucible enlargements will lead to a further exacerbation of these problems.
The document US 2008/0196656 A1 describes crucibles made of quartz ceramics which are provided with several coatings and are suitable for the production of crystalline silicon ingots. The coating of the crucibles is intended to prevent the formation of cracks and fractures in the silicon ingot and to allow extraction of the crystallized silicon ingot without damaging the crucible. These crucibles, however, have a monolithic structure and the thermal conductivities of the bottom and the side walls are identical, resulting in the disadvantages described above. Moreover, the production of such crucibles is very complicated and therefore very expensive. SiO2 crucibles of the prior art are therefore usually destroyed because the initially amorphous silicon dioxide is converted by heat impact into a crystalline state (high temperature cristobalite). This crystalline state is transformed into the so-called low temperature cristobalite during cooling. This transformation is accompanied by a volume change which leads to the destruction of the crucible.
The document JP 2000 351 688 A likewise describes crucibles made of quartz ceramics and their manufacture. The crucibles are suitable for the production of silicon ingots. The crucibles have a curved inner bottom which is bordered by a flat outer bottom. This arrangement is intended to ensure the complete contact of the crucible with a cooling plate to allow a uniform cooling of the crucible bottom. This serves to form a uniform solidification front in the directed crystallization of silicon ingots in the Bridgman method and should enable the production of silicon ingots with an increased height. These crucibles are also constructed monolithically and bring along the above-described disadvantages.
The document US 2011/0180229 A1 describes crucibles, in which a bottom plate is inserted into the frame of the crucible. In this case, a firm connection of the bottom plate to the frame by a form fit and by a further material bond is provided. The crucible frame is therefore not set onto the bottom plate and a separation of the bond between the crucible frame and the bottom plate is labor-intensive. Thus, a quick exchange of the frame is not possible. In addition, the bottom plate must be tailored to the frame. These crucibles are therefore difficult to handle and expensive to produce.
The document DE 10 2011 052 016 A1 describes a kit for carbide ceramic crucibles. Such a kit provides the firm connection of the bottom plate and side elements by form fit and material bond, wherein the frame is also not set onto the bottom plate. An increased thermal conductivity of the bottom plate of such a crucible is not provided. Thus, such a finished crucible basically corresponds to the design of a monolithic crucible and brings about the corresponding above-mentioned disadvantages.
The document DE 10 2012 102 597 A1 describes a process for producing a directionally solidified material body. In this case a crucible is prepared such that its bottom is covered with a plurality of thin monocrystalline seed crystal plates. This should lead to a directional crystallization of quasi-monocrystalline metal or semi-metal bodies. The crucibles described are also constructed monolithically and bring about the disadvantages described.
Thus, there is a need to improve the melting and crystallization so that a better control of the direction of crystal growth is enabled while improving and accelerating the establishment of the crystallization rate. Furthermore, the process should be optimized so that the costs of the crucible are minimized. It is the object of the present invention to provide these improvements.